The present invention relates to ink jet compositions and printing processes. More specifically, the present invention is directed to ink compositions suitable for use in thermal ink jet printing processes. One embodiment of the invention resides in a thermal ink jet ink composition comprising a dye, a liquid medium, and a surfactant such as those selected from the group consisting of polyoxyalkylated ethers, anionic bitail fluoro thio alkyls, alkyl aryl sulfonates, alkyl amine quaternary salts, and mixtures thereof.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The other type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in publications such as U.S. Pat. Nos. 4,601,777; 4,251,824; 4,410,899; 4,412,224 and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Ink jet inks are also known. For example, U.S. Pat. No. 4,508,570, the disclosure of which is totally incorporated herein by reference, discloses an aqueous ink for ink jet printing which comprises a water-soluble direct dye and/or acid dye, a polyhydric alcohol and/or an alkyl ether thereof, water, and at least one water-soluble non-ionic surface active agent selected from a specified polyoxyethylene alkyl amine, a specifed polyoxyethylene alkyl phenyl ether, and a specified polyoxyethylene alkyl ether. In addition, U.S. Pat. No. 4,623,689, the disclosure of which is totally incorporated herein by reference, discloses an ink jet ink containing aqueous colored polymers which comprise a homopolymer of an ethylenically unsaturated sulfonic acid or its salt or a copolymer of an ethylenically unsaturated sulfonic acid or its salt with another ethylenically unsaturated monomer, wherein the homopolymer or copolymer is dyed with a basic dye and has a lowest film-forming temperature of not higher than 35.degree. C. Further, U.S. Pat. No. 4,627,875, the disclosure of which is totally incorporated herein by reference, discloses a recording liquid suitable for ink jet recording which comprises C.I. Acid Red 8 as the recording agent and a liquid medium comprising at least a member selected from polyethylene glycol, polyethylene glycol mono methylether, and a mixture thereof; a member selected from diethylene glycol, sulfolane, and a mixture thereof; a member selected from N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, and a mixture thereof; and water.
In addition, the use of surfactants in other applications is known. For example, U.S. Pat. No. 3,948,668 discloses a printing ink containing from about 0.01 percent to 1.0 percent by weight of a fluorocarbon surfactant containing a fluorocarbon moiety selected from CF.sub.3 (CF.sub.2).sub.3, CF.sub.3 CF(CF.sub.3)O--, and CF.sub.3 CF(CF.sub.3)CF.sub.2 --, wherein the surfactant contains at least 25 percent fluorine by weight. In addition, U.S. Pat. No. 4,139,509 discloses a household starch composition prepared with a surface active agent, examples of which are provided in column 5, line 50 to column 7, line 21 and in column 9, line 58 to column 11, line 7.
Although the known ink jet ink compositions are suitable for their intended purposes, a need continues to exist for new and improved thermal ink jet inks. There is also a need for thermal ink jet inks that are non-toxic, non-carcinogenic, and non-mutagenic. In addition, a need exists for thermal ink jet inks with improved latency times, wherein latency is a measure of the amount of time for which the jet can be stopped and later started without clogging. Further, a need continues to exist for thermal ink jet inks with acceptable viscosity and surface tension values. A need also exists for thermal ink jet inks that do not contaminate or cause deterioration of the ink jet printer heater. In addition, there is a need for thermal ink jet inks with reduced drying times. A need also exists for thermal ink jet inks that exhibit reduced bleeding or feathering on specialty papers, plain papers, and transparencies. Further, there is a need for thermal ink jet inks that exhibit good lightfastness, waterfastness, and print density and that result in little or no formation of residual deposits on the printer heater. There is also a need for thermal ink jet inks that exhibit good mixing of primary colors to generate mottle-free images of the desired color, independent of the order in which the primary color inks are applied to the substrate. Mottle is observed when inks of two different colors are mixed to form an image of a secondary color and the inks become segregated by color as a result of their rejecting each other because of differences in surface tensions. Inks of the present invention can be employed to form mottle-free images of secondary colors, and color quality remains constant regardless of the order in which the colored inks are applied to the printing substrate.